PROJECT SUMMARY/ABSTRACT Individuals with cerebral palsy (CP) experience a progressive decline in ambulatory ability, beginning in adolescence and into early adulthood. We propose that impaired muscle power generation is the key limiting factor affecting walking activity and the ability to physically participate in day to day life. To optimize ambulatory function, it is also essential to practice the task of walking, known as task-specific training, in order to learn how to generate muscle power during walking. However, treadmill training for children with CP should be modeled on the activity pattern of typically developing (TD) youth, which consists of intervals of high intensity activity. Therefore, the objective of this proposal is to determine the effect of lower extremity Power Training combined with Interval Treadmill Training (PT3) on functional walking capacity and community-based activity and participation in children with CP. To identify key muscular mechanisms with this type of training, we will examine quadriceps muscle performance and architecture. We hypothesize that remediating the most pronounced muscle performance impairment (i.e., muscle power) with power training combined with a task- specific approach to walking that is developmentally appropriate will have a significant effect on walking capacity and performance. This hypothesis will be tested by the following specific aims. Aim #1: determine the immediate and retention effects of power training combined with interval treadmill training (PT3) on functional walking capacity in ambulatory children with CP. Forty-eight ambulatory participants with CP (10 to 17yrs) will be randomized to receive either PT3 or an equivalent dosage of traditional approaches (strength training combined with traditional treadmill training) for 24 sessions, 3 times per week for 8 weeks. Walking outcomes will be collected at baseline, immediately post-treatment, and at 2 and 6 months post. Aim #2: quantify the effects of treatment on in vivo muscle architecture and muscle performance measured with ultrasound imaging and dynamometry. We hypothesize that muscle power generation is impaired and is responsible for limitations in walking activity, and that PT3 will result in significant improvements in muscle power that will be explained partly by increases in cross-sectional area and fascicle length. Aim #3: using coordinate data from global position system (GPS) combined with accelerometry, we will directly measure the effects of treatment on community-based walking activity and participation captured in real-world settings. This project is innovative because it represents a paradigm-shift in current clinical rehabilitation practice in that PT3 targets muscle power deficits specifically to drive changes in clinic and community-based walking activity that will be measured with novel technology during day to day life. The proposed research is significant because it will be the first step in a line of research that is expected to ameliorate the documented deterioration in walking ability that begins in adolescence and early adulthood in CP. Knowledge generated from this proposal has relevance for the overall health, mobility, and physical activity in individuals with CP across the life span.